1 Sustainable landfill leachate treatment using refuse and pine bark 1 as a carbon source for bio-denitrification 2 3 R.R. Frank a,b , C. Trois a and F. Coulon a,b 4 5 a Centre for Research in Environmental, Coastal and Hydrological Engineering 6 (CRECHE), School of Engineering, University of KwaZulu-Natal, Howard College 7 Campus, Durban, 4041, South Africa. 8 b School of Energy, Environment and Agrifood, Cranfield University, MK43 0AL, 9 United Kingdom 10 11 Corresponding Author: Prof Cristina Trois, [email protected]12 Article type: ORGINAL RESEARCH ARTICLE 13 14 Funding 15 The work has been supported by Durban Solid Waste and the National Research 16 Foundation (South Africa). 1 17 18 * Correspondence details: Prof Cristina Trois. [email protected]
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Sustainable landfill leachate treatment using refuse and pine bark1
as a carbon source for bio-denitrification2
3
R.R. Frank a,b, C. Trois a and F. Coulon a,b4
5
a Centre for Research in Environmental, Coastal and Hydrological Engineering6
(CRECHE), School of Engineering, University of KwaZulu-Natal, Howard College7
Campus, Durban, 4041, South Africa.8
b School of Energy, Environment and Agrifood, Cranfield University, MK43 0AL,9
Published by Taylor & Francis. This is the Author Accepted Manuscript issued with: Creative Commons Attribution Non-Commercial License (CC:BY:NC 3.0). The final published version (version of record) is available online at 10.1080/09593330.2014.989279. Please refer to any applicable publisher terms of use.
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Abstract: Raw and 10-week composted commercial garden refuse materials and pine bark mulch1
were evaluated for their potential use as alternative and sustainable sources of carbon for landfill2
leachate bio-denitrification. Dynamic batch tests using synthetic nitrate solution of 100, 500 and3
2000 mg NO3 L-1 were used to investigate the substrate performance at increasing nitrate4
concentrations under optimal conditions. Further to this, sequential batch tests using genuine5
nitrified landfill leachate with a concentration of 2000 mg NO3 L-1 were carried out to evaluate6
substrates behaviour in the presence of a complex mixture of chemicals present in leachate.7
Results showed complete denitrification occurred in all conditions indicating that raw and8
composted commercial garden refuse and pine bark can be used as sustainable and efficient9
media for landfill leachate bio-denitrification. Of the three substrates, raw garden refuse yields10
the fastest denitrification rate followed by 10-week composted commercial garden refuse and11
pine bark. However the efficiency of raw commercial garden refuse was lower when using12
genuine leachate, indicating the inhibitory effect of components of the leachate on the13
denitrification process. 10-week composted commercial garden refuse performed optimally at14
low nitrate concentrations, while poor nitrate removal ability was found at higher nitrate15
concentrations (2000 mg L-1). In contrast pine bark performance was 3.5 times faster than the16
composted garden refuse at higher nitrate concentrations. Further to this, multi-criteria analysis of17
the process variables provided an easily implementable framework for the use of waste materials18
as an alternative and sustainable source of carbon for denitrification.19
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Keywords: denitrification; leachate treatment; carbon source; pine bark; commercial garden21
refuse.22
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1. Introduction1
South Africa produces 108 million tonnes of waste per annum, of which 98 million2
tonnes are sent to landfill sites. [1] This significant amount of waste contains a3
large proportion of bioreactive wastes, which produce mainly gas and wastewater4
known as leachate. [2-3] Leachate treatment and disposal is one of the biggest5
issues during solid waste management practices. Leachate has very high strength6
regarding to pH, Chemical Oxygen Demand (COD), Biological Oxygen Demand7
(BOD), ammonia, chloride, colour, odour, and heavy metals. If it is not collected8
carefully and not discharged safely, leachate has the ability to cause major9
environmental impacts as well as affect human health due to its high toxicity. The10
major concern associated with leachate is its ammonia content, which can reach11
levels of up to 1000 mg L-1. [3-6] Leachate can contaminate ground and surface12
water resources, which can consequently affect potable water supply. Furthermore,13
it can affect biological systems and ecological communities of many fauna and14
flora exposed to contaminated water. [4,7,8] There are typically few wastewater15
treatment facilities in developing countries due to the high cost of treatment and16
lacking environmental pollution control laws and enforcement. [8] The ammonia in17
leachate can be treated by biological nitrification. [9,10] Such a process approach18
has been adopted at the Mariannhill landfill site (LFS) (Durban, South Africa),19
which receives between 550-700 tonnes of municipal solid waste per day [10] and20
produces and nitrifies approximately 30 m3 of leachate per day. The leachate21
treatment plant operates by aerobically converting ammonia to nitrites, and then to22
nitrates.23
Nitrates can still have significant environmental and human health implications.24
Nitrates can lead to adverse eutrophication in aquatic environments. [11] Nitrate25
levels > 45 mg L-1 can also affect human health [12] through ingestion of nitrate-26
4
containing water or vegetables, causing among others; abdominal pains, diarrhoea,1
vomiting, diabetes, birth defects, infant mortality, hypertension and respiratory2
tract infections. [13] Therefore a denitrification step is required to reduce nitrate3
levels to acceptable discharge limits. In South Africa, the nitrate discharge limit set4
by the Department of Water Affairs and Forestry is 15 mg NO3 L-1. [14] The nitrate5
concentration of the effluent from the nitrification sequencing batch reactor (SBR)6
installed at the Mariannhill landfill site ranged between 285 and 1425 mg L-1 in7
2011/12. The SBR effluent is currently recirculated into the landfill through use as8
a dust suppressant. Closure of the landfill site is expected in 2022 at which point9
recirculation will no longer be a viable treatment option. [15] The use of biological10
denitrification, in the form of a biological anaerobic filter bed operated in a flowing11
system, will be adopted at the landfill site as this is believed to be one of the most12
promising methods of nitrate removal. [3]13
Biological denitrification is the process by which oxidised nitrogenous14
compounds such as nitrates or nitrites are reduced to nitrogen gas under anoxic15
conditions through the assistance of a diverse group of bacteria. [3,16,17] The16
biological denitrification process typically follows a nitrification step whereby17
ammonia and much of the organics are removed. [18] There is thus a deficiency of18
carbon essential for denitrification. As a result, an external carbon source is19
required as an electron donor in order for microorganisms to survive. [18]20
Typically methanol, glucose, ethanol, propionic and acetic acid are commonly21
used as they are easily biodegradable. [19-21] These carbon sources, however, are22
expensive which consequently restricts their viability in full-scale application. [21]23
The use of waste materials as a carbon source in denitrification has been24
researched for over 20 years. [3,20,22-25] It has the dual benefit of removing25
wastes from the waste stream, diverting it from landfill sites, and use as an26
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alternative carbon source. It is therefore an economically and environmentally1
sustainable carbon source alternative. Alternative carbon sources from waste2
materials that have been found to successfully denitrify leachate include tree barks,3
sawdust, corncobs, wood chips, newspapers, yeast, whey and compost. [20,23].4
Many of these alternative carbon sources have shown denitrification rates and5
chemical oxygen demand:nitrogen ratios (COD:N) comparable to traditional6
chemicals such as methanol and acetic acid. [26] There is, however, still a need to7
continue identifying feasible alternative carbon sources in terms of cost,8
availability and denitrification efficiency in order to continue the development of9
sustainable nitrate removal solutions. [26] Trois et al. [3,21] previously10
investigated the use of composted garden waste and pine bark (PB) as an11
alternative and low-cost carbon source for supporting biodenitrification as they12
are found in high quantities in South African landfills. They demonstrated that13
complete nitrate removal was achievable and provided insights into the key14
microbes involved in the biodenitrification process. However, little information on15
the chemical characterisation during the denitrification process was provided.16
Therefore, the main objectives of this study were to (1) investigate the feasibility of17
using raw commercial garden refuse (raw CGR), 10 week composted commercial18
garden refuse (CGR 10) and pine bark as sustainable alternative carbon sources for19
the denitrification of treated landfill leachate at an initial nitrate concentration of up20
to 2000 mg NO3 L-1; (2) characterise the substrates performance against the nitrate21
concentrations load using synthetic and genuine leachate and (3) provide a decision22
support tool to inform future treatment strategies.23
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2. Materials and Methods25
2.1. Substrate selection26
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Substrates tested were raw CGR, CGR composted for 10 weeks and PB. The CGR1
substrate was sourced from the waste stream of the Bisasar Road Landfill site in2
Durban, South Africa. The CGR substrate, which was made up of mainly thin twigs3
and leaves, went through an onsite chipper which reduced the chip size to smaller4
than 5 cm in length. It was subsequently stored in an onsite pile and collection of5
sample happened within days of the chipping process. To obtain the CGR 10,6
composting of the raw CGR was conducted on site for 10 weeks through a turned7
windrow technology. The PB was obtained from the MONDI paper company in8
South Africa. They were prepared as wood chips with a length approximately 3-9
5cm in size.10
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2.2. Substrate and leachate characterisation12
Substrate and leachate characterisation methodology was conducted according to13
standard analytical methods as published by the American Public Health14
Association. [27]15
Characterisation was conducted on both the solid and eluate fractions of the16
substrate. The eluate was attained by immersing the substrate in distilled water for17
24 hours at a liquid:solid (L/S) ratio of 10:1 by weight. This enabled optimal liquid18
to solid contact.19
Characterisation tests which were conducted on the solid substrates included20
moisture content (w), total solids (TS), volatile solids (VS), respiration index (RI7),21
total carbon (TC), total nitrogen (TN) and carbon to nitrogen ratio (C/N).22
Characterisation tests which were conducted on the eluate samples and leachate23
included: TS, VS, pH, soluble chemical oxygen demand (sCOD), biochemical24